A flat panel display possesses advantages of being ultra thin, power saved and radiation free and has been widely utilized. Present flat panel displays mainly comprise a LCD (Liquid Crystal Display) or an OLED (Organic Light Emitting Display).
The organic light emitting display possesses outstanding properties of self-illumination, no required back light, high contrast, being ultra thin, wide view angle, fast response, being applicable for flexible panel, wide usage temperature range, simple structure and manufacture process and etc., and therefore, it is considered to be a new applicable technology for the next generation flat panel display.
The OLEDs can be categorized as PM-OLED (Passive matrix OLED) and AM-OLED (Active matrix OLED). The AM-OLED panels belong to active display type, and require manufacturing pixel structures in array on the array substrate. As shown in FIG. 1, generally in each of the pixel structures in the present AM-OLED, two Thin Film Transistors (TFTs) sandwich a storage capacitor in-between. Specifically, the first thin film transistor TFT1′ comprises a first gate electrode 210, a first source electrode/a first drain electrode 610 and an etching stopper layer 500, a first semiconductor layer 410, a gate isolation layer 300 sandwiched in-between; the second thin film transistor TFT2′ comprises a second gate electrode 220, a second source electrode/a second drain electrode 620 and an etching stopper layer 500, a second semiconductor layer 420, a gate isolation layer 300 sandwiched in-between; the storage capacitor C′ comprises a first metal electrode 230 formed by the first and the second gate electrodes 210, 220, and a second metal electrode 630 formed by the first source electrode/the first drain electrode, the second source electrode/the second drain electrode 610, 620, and the etching stopper layer 500 and the gate isolation layer 300 sandwiched between the two electrodes. Because the metal material shields the light and blocks the penetration of the light, the first and the second thin film transistors TFT1′, TFT2′ and the storage capacitor C′ must occupy a certain area of the pixel, which leads to a reduced activation area of the pixel, meaning an aperture ratio is reduced. It enormously restricts the usage ratio of the light. Particularly as for the high resolution, bottom emitting AM-OLED, the decrease of the aperture ratio gets even worse, which easily causes problems of the insufficient brightness and over large power consumption.
FIG. 2 is an equivalent circuit diagram of FIG. 1. The first thin film transistor TFT1′ is employed as a single switch thin film transistor. The second thin film transistor TFT2′ is employed as a drive thin film transistor. Specifically, a gate electrode of the first thin film transistor TFT1′ is coupled to a gate drive voltage signal Vgate, and a source electrode thereof is coupled to a data drive voltage signal Vdata, and a drain electrode thereof is coupled to a gate electrode of the second thin film transistor TFT2′; a source electrode of the second thin film transistor TFT2′ is coupled to a drive voltage signal Vdd, and a drain electrode thereof is coupled to an anode of the organic light emitting diode D; a cathode of the organic light emitting diode D is coupled to a ground signal Vss; one electrode of the storage capacitor C′ is coupled to the gate electrode of the second thin film transistor TFT2′, and the other electrode thereof is coupled to the source electrode of the second thin film transistor TFT2′.
The working principle of the circuit is as follows. When the gate drive voltage signal Vgate is present, the first thin film transistor TFT1′ is conducted, and the data drive voltage signal Vdata is inputted to the gate electrode of the second thin film transistor TFT2′ and the second thin film transistor TFT2′ is conducted. The drive voltage signal, Vdd, drives the organic light emitting diode D to display after being amplified by the second thin film transistor TFT2′. When the gate drive voltage signal Vgate disappears, the storage capacitor C′ is a major means to maintain the voltage level of the pixel electrode.